Since yesterday, Dropbox stopped the support for their so called API v1 any only allow access with the new API v2. That means, older apps which did not yet move to the new Dropbox access method cannot use the Dropbox anymore.

Also yesterday I got ready with the new Dropbox API. As usual, it will last 2-3 days for Calypso v4.0.1 to become available in the App Store. Until then, you have to workaround the missing Dropbox support. Download your files with the Dropbox app and share it with Calypso (Open in ...).

BTW.: the upcoming v4.0.1 will fix most of the small and big bugs of the new Calypso v4.